1. Field of the Invention
The present invention relates to a photocatalyst-bearing material usable for environmental cleanup of the air, water, etc. and effective utilization of light energy such as solar energy, and to a method of producing the same.
2. Related Art
When photocatalyst particles are irradiated with a light having an energy of at least the band gap thereof, electrons and holes come into existence in the conduction band and valence band, respectively, of the photocatalyst particles due to light excitation. The electrons produced in the conduction band have a strong reducing power, while the holes produced in the valence band have a strong oxidizing power. Thus, these photocatalyst particles are utilized for the purposes of cleanup of harmful substances, deodorization of malodorous substances, decomposition of water, sterilization, etc. [see Kazuhito Hashimoto, Akira Fujishima, xe2x80x9cApplication of Photocatalytic Reactions to Water Purification,xe2x80x9d Journal of Water and Waste, Vol. 36, No. 10, pp. 851-857 (1994); Sadao Murasawa, xe2x80x9cDeodorization Method Using Photocatalyst,xe2x80x9d Environmental Management, Vol. 32, No. 8, pp. 929-934 (1996); Kazuhiro Sayama, Hironori Arakawa, xe2x80x9cStoichiometric Decomposition of Water over Semiconductor Photocatalyst,xe2x80x9d Catalysts and Catalysis, Vol. 39, No. 3, pp. 252-256 (1997); etc.]. Incidentally, photocatalytic reactions are mainly oxidation reactions basically attributed to holes excited by photons, which reactions usually proceed only on the surfaces of photocatalyst particles and more particularly only on sites thereof irradiated with a light such as ultraviolet rays.
Such photocatalyst particles must be easily handleable, and easily separable and recoverable from a liquid treated therewith in most cases. For this reason, photocatalyst particles are generally used in such a state that they are supported or coated on a carrier such as a flat plate, a granular material, a honeycomb structure or a three-dimensional reticular structure. Methods of supporting photocatalyst particles on a carrier include one involving precipitating and fixing photocatalyst particles on a carrier [see Japanese Patent Laid-Open No. 5-96181 published on Apr. 20, 1993; and Masayuki Murabayashi, Kazuo Okamura, xe2x80x9cDegradation of Chloroorganic Compounds by Using Fixed Photocatalyst,xe2x80x9d Journal of Water and Waste, Vol. 36, No. 10, pp. 877-882 (1994)], a sol-gel method involving supporting a photocatalytic chemical substance produced on a carrier by heat decomposition of an organic compound such as titanium tetraisopropoxide [see Katsuhiko Yoshida, Kazuo Okamura, Koji Hirano, Kiyoshi Iguchi, Kiminori Itoh and Masayuki Murabayashi, xe2x80x9cPhotocatalytic Degradation of Trichloroethylene in Water by Using Thin-film TiO2 Prepared by Sol-Gel Process,xe2x80x9d Journal of Japan Society on Water Environment, Vol. 17, No. 5, pp. 324-329 (1994)], and one involving preliminary dispersion of photocatalyst particles in a carrier material [see Japanese Patent Laid-Open No. 8-208414 published on Aug. 13, 1996].
The method involving precipitating and fixing photocatalyst particles on a carrier leaves much to be desired as these particles are readily released, or dislodged, so that the total area of particles wherein photocatalysis can be manifested is decreased in keeping with an increasing number of sites released of photocatalyst particles, thereby lowering the reaction efficiency. The sol-gel method is so complicated in preparation operations as to be unfit for mass production, and cannot give rise to a sufficient activity of photocatalyst because the amount of supported photocatalyst is limited. Further, in this method, a heat-resistant material (glass, metal or the like), which is difficult to handle and expensive, must disadvantageously be used as the carrier material because heating up to at least 300xc2x0 C. must usually be done for heat decomposition of the organic compound. The method involving preliminary dispersion of photocatalyst particles in a carrier material is gravely wasteful because photocatalyst particles are unnecessarily buried inside the carrier.
An object of the present invention, which has been made in view of the foregoing various shortcomings of the prior art, is to provide a photocatalyst-bearing material capable of exhibiting a high photocatalysis for a long period of time, and an inexpensive method of producing the same.
As a result of intensive investigations with a view to doing away with the foregoing shortcomings of the prior art, the inventors of the present invention have found out that bearing particles (B) capable of photocatalysis in and on surface portions of a carrier (A) of thermoplastic polymer by fusion bonding enables the particles (B) to be firmly borne, or supported, on the carrier (A) in such a state that multiple particles (B) are stacked in directions perpendicular to the surfaces of the carrier (A). The present invention has been completed based on this finding.
Specifically, the present invention provides a photocatalyst-bearing material characterized in that particles (B) capable of photocatalysis are fusion-bonded to surface portions of a carrier (A) of thermoplastic polymer in such a state that multiple particles (B) are stacked in directions perpendicular to the surfaces of the carrier (A); and a method of producing a photocatalyst-bearing material, characterized by comprising mixing and contacting a carrier (A) of thermoplastic polymer with particles (B) capable of photocatalysis to fusion-bond the particles (B) to surface portions of the carrier (A) in such a manner that multiple particles (B) are stacked in directions perpendicular to the surfaces of the carrier (A).
The photocatalyst-bearing material of the present invention, which is different from a simple mixture of the carrier (A) with particles (B), is a material having particles (B) borne in and on surface portions of the carrier (A) by fusion bonding, and maintaining a state that particles (B) are partly exposed from the surfaces of the carrier (A) [see FIGS. 1 and 2]. Since multiple particles (B) are stacked in and on a surface portion of every grain of the carrier (A), release, or dislodgment (exfoliation), of some particles (B), even when brought about because of deterioration of the surface of that grain of the photocatalyst-bearing material, lets particles (B) thereunder be sequentially exposed from the surface. Thus, the photocatalyst-bearing material of the present invention can continuously maintain the photocatalysis thereof over a long period of time. Moreover, the method of producing a photocatalyst-bearing material according to the present invention is simply and easily operable to be favorably usable for mass production thereof.
The process of formation of the foregoing photocatalyst-bearing material by the method of producing a photocatalyst-bearing material according to the present invention will be described to be as follows: Surface portions of the carrier (A) are fused, or molten, by heating. Simultaneously, the carrier (A) and particles (B) are mixed and stirred together to fusion-bond some particles (B) to the molten surfaces of the carrier (A). Upon further heating, some melt of the thermoplastic polymer of the carrier (A) is oozed out from between particles (B) fusion-bonded to the surfaces of the carrier (A), thereby further fusion-bonding some other particles (B) to the oozed-out melt of the thermoplastic polymer of the carrier (A). According to the foregoing mechanism that is repeated, multiple particles (B) are stacked and borne on the carrier (A) to produce the photocatalyst-bearing material of the present invention.
The shapes of the carrier (A) and the photocatalyst-bearing material of the present invention may be arbitrary, but are preferably substantially spherical or disk-like from the viewpoint of simplicity of the production procedure. Substantially spherical ones are preferred in respect of handleability, while substantially disk-like ones are preferred because the exposed surface areas thereof are large. The size of the photocatalyst-bearing material of the present invention is not particularly limited, but may be set arbitrarily. For example, where the material is substantially spherical, the average grain size thereof may be 0.1 mm to 30 mm, and is preferably 0.5 mm to 10 mm, more preferably 1 mm to 5 mm.
Examples of the thermoplastic polymer usable in the present invention include olefin homopolymers such as polyethylene and polypropylene, olefin copolymers (copolymers of olefins), copolymers of an olefin(s) with other polymerizable monomer(s), polyvinyl chloride, polyvinylidine chloride, polystyrene, poly(meth)acrylates such as polymethyl methacrylate, polyamides, and polyesters such as polyethylene terephthalate and polyethylene naphthalate. Among them, thermoplastic polymers especially preferred for use as the material of the carrier (A) in respect of capability of easily and firmly bearing particles (B) in such a manner that multiple particles (B) are stacked in directions perpendicular to the surfaces of the carrier (A) include olefin homopolymers, olefin copolymers, and copolymers of an olefin(s) with other polymerizable monomer(s). Herein, examples of suitable olefins include ethylene, propylene, butenes, hexenes, 4-methylpentene, and octenes. Other polymerizable monomers include alicyclic monoenes such as norbornene and cyclopentene; dienes such as butadiene, isoprene, cyclopentadiene, dicyclopentadiene, hexadiene, and octadiene; vinyl esters such as vinyl acetate, vinyl propionate, vinyl benzoate, vinyl lactate, and vinyl monochloroacetate; unsaturated carboxylic acids such as acrylic acid, methacrylic acid, vinylacetic acid, crotonic acid, maleic acid, itaconic acid, and fumaric acid; alkyl esters, alkali metal salts, alkaline earth metal salts, ammonium salts and acid anhydrides of such unsaturated carboxylic acids; unsubstituted or substituted (meth)acrylamides such as acrylamide, methacrylamide, and N-methylacrylamide; acrylonitrile; methacrylonitrile; sulfonic group-containing monomers such as p-styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid; phosphonic group-containing monomers such as allylphosphonic acid and vinylphosphonic acid; N-vinylpyrrolidone; N-vinylformamide; acrolein; vinyl chloride; vinylidene chloride; chloroprene; ethylene fluoride; and styrene. Examples of especially preferred thermoplastic polymers include low-density polyethylene, high-density polyethylene, polypropylene, ethylene-butene-1 copolymers, ethylene-hexene-1 copolymers, ethylene-propylene copolymers, ethylene-octene-1 copolymers, ethylene-acrylic acid copolymers, ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, poly(4-methylpentene-1), ethylene-propylene-diene copolymers, and ethylene-maleic anhydride copolymers. Polyethylene is a thermoplastic polymer that is especially advantageous in cost. Such thermoplastic polymers may be used either alone or in the form of a blend of at least 2 kinds thereof. At least one olefin homopolymer or copolymer, if contained as the main component, may be blended with at least one of polymers other than the above preferred thermoplastic polymers and/or at least one of inorganic fillers. Examples of such blendable polymers include thermoplastic resins such as polyamide resins, polyester resins, polyester or polyamide thermoplastic elastomers, polysulfones, ABS, MBS, polyether-imides, polyether ether ketones, polycarbonates, polystyrene, polyphenylene ethers, and polyphenylene sulfides; thermosetting resins such as phenolic resins, epoxy resins, urea resins, unsaturated polyester resins, and polyimide resins; and synthetic rubbers such as styrene-butadiene rubbers, butadiene rubber, isoprene rubber, acrylonitrile-butadiene rubbers, chloroprene rubber, urethane rubbers, EPM, EPDM, silicone rubbers, and fluororubbers. Examples of inorganic fillers include fibrous fillers such as glass fibers, potassium titanate whiskers, and carbon fibers; substantially platy fillers such as mica and talc; and substantially spherical fillers such as calcium carbonate, carbon black, ferrite, and zeolite.
The melting temperature of the thermoplastic polymer is preferably in the range of 50 to 150xc2x0 C. for production of the photocatalyst-bearing material according to the method of the present invention. Since the tolerable range of the melting temperature may be greatly varied depending on the kind, material, etc. of the thermoplastic polymer, however, it is not limited to the above preferable range. Herein, the melting temperature is measured according to JIS K7121. When the melting temperature is lower than 50xc2x0 C., there may be a fear of failure in maintaining the shape of the photocatalyst-bearing material because the temperature of an object of treatment such as water may approach too close to the melting temperature though it depends on reaction conditions such as the site for installing a reactor using the photocatalyst-bearing material, and the reaction temperature. On the other hand, when the melting temperature exceeds 150xc2x0 C., heating means such as a drum may be limited to be unfit for industrial mass production in many cases.
Examples of particles (B) usable in the present invention include particles of photocatalytic substances such as titanium dioxide, strontium titanate, zinc oxide, iron oxides, zirconium oxide, niobium oxide, tungsten oxide, tin oxides, cadmium sulfide, cadmium telluride, cadmium selenide, molybdenum sulfide, and silicon. Among them, at least one kind of particles can be chosen for use. Preferred is titanium dioxide, which is capable of manifesting an especially excellent photocatalytic performance. Crystal structures of titanium dioxide include anatase type and rutile type. Anatase titanium dioxide is usually used because of a higher photocatalysis. When it is used together with an oxidizing agent, such as hydrogen peroxide, which is capable of forming hydroxy radicals having a strong oxidizing function, however, rutile titanium dioxide may sometimes exhibit a high photocatalysis. Therefore, rutile titanium dioxide is not excluded in the present invention. Use may be made of particles (B) having their surfaces dotted, or loaded, with a metal such as platinum, rhodium, ruthenium or nickel, or an oxide or hydroxide of such a metal. In this case, the photocatalytic efficiency can be improved even if the amount of the dotting substance is extremely small. Particles (B) may have their surfaces dotted, or loaded, with a substance having a light-storing function. Suitably usable examples of such a light-storing substance include those comprising a sulfide, sulfate, silicate or like of alkaline earth metal as the main component, and lead, manganese, bismuth or the like added thereto as an activator. Specific examples of them include BaSO4/Pb, CaSiO3/Pb, and CaS/Bi, which may be used either alone or in combination of at least 2 kinds thereof. The light-storing substance, which is generally called a fluorescent substance, a luminous substance or the like, is a substance capable of once converting the energy of visible light, ultraviolet rays, radiation or the like into a chemical energy for energy storage, and then emitting that chemical energy in the form of a light energy at any time. When this substance is dotted, or borne, over particles (B), the efficiency of light utilization may be improved.
Although the amount of particles (B) borne on the carrier (A) cannot be specified because it varies or is varied largely depending on the kind of photocatalyst particles, the kind of thermoplastic polymer, etc., it is preferably 0.1 to 80 wt. %, more preferably 1 to 50 wt. %, based on the total weight of carrier (A)+particles (B). When it is smaller than 0.1 wt. %, particles (B) are liable to hardly cover the whole surfaces of the carrier (A). On the other hand, when it exceeds 80 wt. %, there arises only an increase in the amount of photocatalyst particles (B) unnecessarily buried inside the photocatalyst-bearing material. This is not so meaningful.